When eye muscles that control the shape of the crystalline lens of a human eye are fully relaxed, a healthy eye tends to focus far away, theoretically “at infinity”. Accommodation is the process by which eye muscles tense in order to change the shape of the crystalline lens, and thereby to change the distance at which the eye is focused.
It is desirable, when testing for refractive aberration of an eye, to control the accommodation (or inversely, the relaxation) of the eye. In many cases, during the testing, it is desirable for the eye muscles to be fully relaxed, reaching the furthest point the testing eye can focus.
Nearsighted individuals (myopia) cannot focus at infinity. Their furthest point of focus is before infinity. For example, a 1D (1 diopter) myope can focus at maximum of 1 meter. Farsighted individuals (hyperopia) can focus optically “beyond infinity”. Placing the eye focus at that point is desired to measure the correct degree of hyperopia.
Convergence is the simultaneous inward movement of eyes toward each other (e.g., when children cross their eyes in jest). Convergence impacts accommodation. Parallel eyes (looking straight ahead with no convergence) drive relaxation of the lens. Converged eyes drive accommodation (focus) to the converged point, affecting the readings of any refractive measurement device.